Devices of this type have already been proposed, with the aim of simplifying the work of laboratories and preventing the risks of contamination and inaccurate assaying. In particular, a one-use device for biological tests is described in International Patent Application PCT/US No. 85.00870. This device comprises, on the one hand, an element for immobilizing the molecules, which element is porous and coupled to a sensor of the molecules to be immobilized, and on the other hand, an absorbent material in contact with the lower surface of the immobilizing element. This known device works as follows. The medium to be tested, which contains the molecules to be assayed, is poured over the upper surface of the immobilizing element, diffuses therethrough and, by capillarity, into the absorbent material. The molecules to be assayed are retained by the sensor on the immobilizing element. Thus, the immobilizing element in some way filters the molecules to be assayed, and the absorbent material has the role of speeding up the flow of the medium to be tested through that filter.